This invention relates to a method of tinting a plastic article; the invention is especially concerned with tinting plastic optical lenses, especially ophthalmic lenses, for use in spectacles and other eye wear, to provide a colour tint or ultraviolet (UV) light transmission inhibiting tint.
Tinting of optical lenses for spectacles and sunglasses is widely employed either to apply an aesthetic, fashion oriented coloured tint to eye wear or to apply a tint which functions to block or inhibit transmission of ultraviolet light in eye wear, while additionally providing a desired aesthetic or fashionable appearance.
Current techniques for tinting optical lenses involve immersing the lens in a bath comprising solution or dispersion of a tinting dye or pigment which is heated to a temperature typically of 75 to 85xc2x0 C. and maintained well below boiling temperature. The heating is achieved by heat exchange, wherein pots containing the tinting dye or pigment are surrounded by a heat transfer fluid which may be a silicone oil or other heat transfer oil. Typically it takes about 60 minutes to heat the bath from room temperature to 75 to 85xc2x0 C. by this conventional technique. The lenses are inserted into the hot bath, typically by means of a lens holder which maintains the lens in an upright or vertical disposition.
As the temperature of the lens rises in the hot bath, the surface pores of the lens open and dye or pigment penetrates to effect tinting. Opening of the surface pores and initial tinting typically takes 5 minutes, but in order to achieve a dark sunglass tint colour, the lens typically needs to be maintained immersed in the hot bath for 15 minutes or more, and in particular up to 45 minutes for a dark tint.
Different optical plastics are employed in lens manufacture. Some optical plastics are tintable, having surface pores which will receive a tinting dye or pigment, for example, diethylene glycol bis(allyl carbonate) known as CR-39. Other optical plastics such as polycarbonates are not-tintable or are difficult to tint.
In some cases optical lenses are provided with a thin, hard surface coating, typically about 2 microns, these hard surface coatings permit tinting but conventional tinting techniques require long tinting times, for example, several hours. Hard surface coatings may be, for example, of vinyl polyester or polysiloxane.
These hard surface coatings do permit tinting of optical lenses of non-tintable plastic, but as indicated above long tinting times are required. These hard surface coatings are also employed, in some cases, on otherwise tintable lenses such as CR-39 lenses, rendering such lenses more difficult to tint.
These prior techniques employ low energy dye pigments available for optical use, which pigments disintegrate within a few hours if exposed to temperatures of 95xc2x0 C. or higher.
U.S. Pat. No. 5,560,751 describes another conventional technique in which a thin liquid coating of a tinting solution is formed on a surface of a lens by spinning the lens at 1000 to 2000 rpm while applying the tinting solution dropwise to the lens surface, whereafter the resulting coated lens is heated causing the dye in the coating to be absorbed.
Canadian Patent Specification 2,095,703 describes a method for producing a photochromic plastic lens in which the lens is immersed in a high boiling organic solvent bath containing the dye and exposed to microwave heating, typically for about 6 minutes, whereafter the lens was allowed to soak in the hot bath for about 15 minutes.
The use of organic solvents is costly and use of hot solvents requires special handling facilities and equipment to avoid or minimize escape of vapors of the hot solvents into the atmosphere with the consequent hazards to the environment and to personnel involved in the tinting operation.
It is thus desirable to develop tinting techniques which avoid the use of hot organic solvents, while at the same time shortening the time required for tinting and avoiding the need for special or costly equipment.
The invention seeks to provide a tinting method in which tinting is achieved in short times.
The invention further seeks to provide a tinting method which avoids the need for costly equipment or handling facilities and employs water-based materials.
In accordance with the invention there is provided a method of tinting a tintable plastic article comprising: i) immersing a tintable plastic article in an aqueous dispersion of a tinting agent; ii) exposing said aqueous dispersion with said immersed article to microwave radiation to bring said dispersion to ebullition; iii) maintaining said ebullition for a time period of at least about 2 seconds with transfer of tinting agent from said dispersion to said article to effect tinting, iv) removing the resulting tinted article from said dispersion; and v) rinsing the tinted article with water to remove residual dispersion.
i) Plastic Articles
The invention is applicable to tintable plastic articles generally, especially small articles, but has particular application to plastic optical lenses employed in eye wear, more especially, spectacles, sunglasses and protective eye wear, such as eye shields and goggles.
It is to be understood that xe2x80x9ctintable plastic articlesxe2x80x9d in the context of the invention, contemplates articles such as lenses which are either of a plastic which is itself tintable, or have a surface coating which is tintable.
The tinting method may be employed to provide a tint of a desired colour, for aesthetic or fashion reasons, or to provide a protective tint effective to reduce, inhibit or block transmission of ultraviolet light through the lens, or for both of these functions.
Typical plastics for eye wear are well established in the eye wear industry and include poly(methyl methacrylate), cellulose acetates, polyvinyl chloride, polyurethanes, polycarbonate and diethylene glycol bis(allyl carbonate).
Within this class of plastics, diethylene glycol bis(allyl carbonate) referred to in the trade as CR-39 is widely employed as a tintable optical lens, both for use in producing a tinted lens of a desired colour and shade, and also for producing lenses with a protective UV tint.
Polycarbonates are also widely employed as optical lens plastic.
ii) Tinting Method
a) Pretreatment
The method of the invention is applied to clean lenses and, if necessary, the lenses are subjected to a preliminary cleansing operation. If lenses are being re-tinted or it is desired to change an existing tint, the existing tint is first removed using a technique similar to that of the invention but with a water-based tint removal solution instead of the aqueous dispersion of dye.
In the tint removal operation the tinted lens is immersed in an aqueous solution of a surfactant effective for removal of the tint under the operating conditions.
The solution of surfactant is first exposed to microwave radiation until the solution boils quietly; the tinted lens is immersed in the hot boiling solution and boiling is maintained to liberate the tinting dye from the lens. The operation is continued until the dye is removed from the lens.
The lens is removed from the solution and washed and the aqueous solution containing liberated tinting agent is reused for removal of tint in other lenses or is discarded.
A clean lens which is to be tinted is suitably surface conditioned by immersing the lens in an aqueous solution of a surfactant which is effective to reduce or lower surface tension on the surfaces of the lens, such that the tinting dispersion uniformly coats the surfaces of the lens when the lens is immersed in the tinting dispersion.
One suitable surfactant for this purpose is that available under the Trade-mark KIRALON-OL from BASF, however, other suitable surfactants can be readily identified by trial and experiment.
The surface conditioning can be carried out at room temperature; suitably the lens is immersed in the aqueous solution of surfactant for 10 to 40, preferably 15 to 30 seconds, removed and rinsed with water.
It is found that if the surface conditioning is not carried, in some cases the tint produced is not homogeneous.
b) Tinting
In order to tint the lens, an aqueous dispersion of a dye or pigment is selected for the desired colour of tint. The aqueous dispersion is housed in a container and the lens is immersed in the dispersion; the lens may be supported by a lens holder which leaves the major faces of the lens fully exposed or may be placed on the floor of the container with a positive side of the lens, i.e., the convex face of the lens uppermost. In this way, both major faces of the lens, i.e., the concave face and the convex face are exposed to the aqueous dispersion; the dispersion and the immersed lens are heated by microwave energy in a microwave oven until the dispersion reaches ebullition, and preferably a quiescent or non-violent boil. After the boiling state is reached, the lens is maintained in the boiling dispersion for a predetermined time to achieve a desired level of tint.
The dye or pigment in the hot dispersion penetrates the surface pores of the lens, and thus dye or pigment is transferred to the lens across the major surfaces of the lens. The surface pores may be in a coating on the surface of the plastic lens or in the surface defined by the plastic from which the lens is manufactured.
Suitably the container housing the dispersion is supported on a turntable in the microwave oven, which defines a rotating platform. Rotation of the turntable supporting the container of dispersion ensures a uniform heating and uniform temperature throughout the dispersion, which in turn results in a uniform transfer of dye to the lens surfaces. Rotation of the turntable is typically at 2 to 10 rpm, more usually 4 to 8 rpm.
It is found preferable to maintain the exposure of the dispersion and lens to the microwave radiation, on a continuous basis, throughout the tinting. Microwave ovens have different heating operations, some of which provide continuous microwave radiation in a heating cycle, and others of which provide microwave radiation in an interrupted or discontinuous manner in which the microwave radiation is interrupted or discontinued periodically in a heating cycle.
It has been found that use of a continuous exposure to microwave radiation provides faster tinting than the use of an interrupted or discontinuous exposure. Furthermore, the use of continuous microwave radiation is found to be especially important in the case of lenses which are more difficult to tint, such as high index or polycarbonate lens. In these cases the use of continuous microwave radiation resulted in faster tinting and darker tints than could be achieved with discontinuous microwave radiation.
The time for the aqueous dispersion to reach boiling at particular microwave oven operating parameters for a particular microwave oven are predetermined by trial. Likewise the time of immersion of a lens in the dispersion, to reach a desired level of tinting is predetermined by trial.
Immersion times for different levels of tinting may be predetermined by trial for specific operating parameters of a specific microwave oven to provide a table of immersion times and tinting levels, to which reference may be made when a lens is to be tinted.
In general a typical 800 watt power microwave oven at high power will bring 120 ml of aqueous dispersion, having the lens immersed therein, to boiling temperature in about 60 seconds. The boiling is maintained for a time depending on the level of tinting desired. In the case of an uncoated CR-39 lens, tinting is complete in 2 to 60 seconds. Typically, a 2 second immersion produces a tinted lens with 10% absorption of visible light, a 30 second immersion produces 50% absorption and a 60 second immersion produces 72% absorption. Other levels of absorption are achieved by shortening or lengthening the immersion time, a longer time producing a darker tint.
On completion of the tinting, the microwave heating is discontinued, the lens is removed from the hot dispersion and rinsed with water to remove residual dispersion.
The lens is preferably removed immediately from the dispersion at termination of the heating, and continued soaking in the dispersion is avoided. If the lens is left soaking in the dispersion, the tinting by the hot dispersion will continue, even if at a slower rate so that the desired level of tinting may be exceeded.
The dispersion may be used repeatedly to tint further lenses until it is depleted in dye.
Since the dispersion reaches the boiling stage rapidly, it is not necessary to maintain it at boiling in between tinting operations. Thus by allowing the dispersion to cool in between tinting operations, exposure of the dispersion to high temperatures is reduced whereby the life of the dye is maintained. In addition the short tinting times required by the method of the invention also result in prolongation of the life of the dye in the dispersion.
In the case of lenses of non-tintable plastic and which have a thin, hard coating which permits tinting, for example, polycarbonate lenses or high index lenses, which have a hard coating of, for example, a vinyl ester or a polysiloxane, the tinting may take up to 12 minutes of immersion in the boiling dispersion, but this is still markedly faster than with conventional tinting techniques where tinting requires upwards of 45 minutes and frequently several hours.
Furthermore, employing the method of the invention to apply UV tints of the benzophenone class, it has been found that the tinting dispersion has a significantly longer reheat life than when the tinting dispersion is employed in the conventional methods.
In the conventional method, even though operated at lower temperatures, the benzophenone UV tints are found to degrade significantly after only 6 to 8 cycles of heating, cooling and re-heating; the degradation is such that the dispersion will no longer function and must be discarded. This degradation occurs whether the 6 to 8 cycles are over a short or a long period of time.
Employing the method of the present invention with benzophenone tints, however, the degradation does not occur even after 70 cycles of heating, cooling and reheating, even through higher heating temperatures are involved.
The reason for this result is unclear, but one theory is that the exposure to microwave radiation or the presence of the surfactant contribute to the stability.
In the present invention it is found that the dispersion can be employed until it is substantially depleted in tinting agent, and the tinting agent is not degraded.
The method of the invention achieves tinting in times that are at least 10 to 20 times shorter than in currently employed techniques, and the tinting dispersion can be employed repeatedly until it is depleted of tinting agent.
c) Tinting Dispersion
The aqueous dispersion of tinting agent suitably contains the dye or pigment and a surfactant to stabilize the dispersion.
The tinting agent is an organic dye or pigment, in particular, a high energy tinting dye or pigment or UV dye which withstands temperatures of 90xc2x0 C. to 135xc2x0 C. without significant degradation.
One suitable class of tinting dyes is the azo dyes available under the Trade-mark CIBACET from Ciba-Geigy Dyes Ltd. A suitable class of UV dye is the benzophenones available under the Trade-mark UVINUL of BASF.
Particular UV dyes of this class include 2,2xe2x80x2,4,4xe2x80x2-tetrahydroxy benzophenone, benzophenone-6, benzoresorcinol, oxybenzone and sulisobenzone.
The azo dyes are available in red, blue and yellow and these three primary colours can be employed to make a full range of colours, employing appropriate proportions of the three primary colours.
In the preparation of the aqueous tinting dispersion weighed amounts of a tinting agent or agents are employed to provide a desired colour, and the dyes are blended with water, preferably luke warm water; suitably the dye is added slowly to the water during mixing or blending. An anti-foaming agent may be added as foaming occurs and finally a surfactant is added to stabilize the dispersion. Suitable surfactants include those derived from castor oil, for example, the ethoxylated castor oil surfactants available under the Trade-mark ALKAMULS from Rhone-Poulenc, especially Alkalmuls EP-620 which is castor oil ethoxylated (30).
Typically the dispersion is produced as a concentrate which is diluted for use.